The invention is generally directed to a thermal printer and in particular to a thermal printer which has a biasing mechanism for selectively pressing the thermal head against the platen at various force levels to print on a variety of types of paper including rough bond paper. The widespread use of thermal printers has generated a need for thermal printers, especially serial type thermal printers which are capable of high speed printing and high quality printing.
A thermal printer disclosed in Japanese Laid-Open Utility Model No. 58-29438 utilizes a variable force of the thermal head against thermal paper to change the color density. Two color levels of printing are obtained by using two levels of pressing force. However, this technique is inadequate for a thermal transfer printer.
A thermal printer, disclosed in Japanese Laid-Open Patent No. 60-131264 uses a roller or a humidifying means for increasing the smoothness of paper to be used in a thermal printer. The smoothness of the paper utilized is also detected.
It is known that the pressing force of a thermal head against the paper in a thermal printer, and especially in a thermal transfer printer, makes a great difference in print quality. Particularly when normal paper is used in a thermal printer, a large biasing force, pressing the head against the paper and platen, is required for quality printing. On the other hand, when smooth thermal transfer paper is used in a thermal printer, high speed printing is required. However, when a large biasing force is used, an increase in the load on the motor results in trailing of the printed dots and smearing of the paper due to the heat stored in the thermal head in high speed printing mode with resulting low print quality. The prior art thermal printers suffer from these disadvantages.
It has become necessary for commercial thermal printers to be able to print on normal (non-thermal) paper. However, many of the printers are unable to print on rough types of paper, such as the bond papers widely used in the United States. Where the thermal printers have been adapted to print on the rough paper, mechanical plungers, which are loud, have been utilized to vary the head pressure. As a result, the quiet printing which is a great benefit of the thermal printers is lost.
The prior art thermal printers are also large units which are difficult to handle. In addition, in order to achieve high print quality, in many cases prior art printers have found it necessary to increase the smoothness of the paper to achieve high quality printing. For example, the bond paper commonly used by businesses in the United States has a rough surface. However, the rough surface is changed by the prior art printing to increase the surface smoothness to aid in high quality printing. In addition, the prior art thermal printers process the paper through a humidifier, roller or other mechanism to smooth the paper which results in a decrease in printing speed.
Accordingly, there is a need for a thermal printer which is capable of selectively biasing the thermal head against the paper and platen at at least two levels; prints in both high quality and high speed modes by selecting a pressing force and a corresponding thermal head movements speed; is capable of printing on a variety of types of paper including rough bond paper; and utilizes a quiet mechanism for changing the pressing force of the thermal head to maintain the silent printing associated with thermal printers